The present invention relates to laminated stator and rotor cores for use with electromagnetic machines and more particularly to core configurations including laminations which have disparate physical properties and operational characteristics.
One common motor type is an induction motor. All induction motors include a stator assembly and a rotor assembly. A typical stator assembly includes a stator core which forms a stator cavity and a plurality of stator windings wrapped around the core such that when current passes through the windings, the current creates an electromagnetic field inside the cavity. In addition to providing support to the windings, the core serves as a flux path for stator field flux and hence strengthens the field within the cavity. By varying the stator currents the magnetic field within the cavity is caused to rotate about the cavity.
A typical rotor assembly includes a rotor core and a plurality of rotor bars (e.g. aluminum bars) which are shorted together at their ends by two shorting rings (e.g. aluminum rings) to form a "squirrel cage" around the core, the core and bars configured so as to fit within the stator cavity. The bars and ring are typically radially constrained by rotor tooth tips or rotor slot bridge formed by the rotor core. The rotor is mounted on a shaft for rotation within the cavity, the rotor and stator together forming an air gap. The rotor bars are arranged such that when the stator magnetic field rotates about the stator cavity, the varying field within the cavity induces rotor current within the bars. The rotor current in turn causes a rotor magnetic field within the stator cavity. The stator and rotor fields interact such that, as the stator field rotates about the core, the rotor field is drawn in the same direction and the rotor rotates within the cavity about the axis of the shaft. In addition to providing support for the bars, the rotor core operates as a flux guide for rotor magnetic field flux.
Motors are typically designed with an end use in mind and therefore, each motor must be able to achieve certain minimal operating characteristics within a specific budget. For example, a motor must be able to generate at least a certain minimal amount of torque, drive at least a minimal load and achieve at least a minimal rotational velocity. To this end, core structures are typically designed such that they have at least certain minimal operating characteristics and a specific cost which can be relied upon when configuring other motor components.
In addition to cost, perhaps the two most basic design criteria for cores are size and efficiency, efficiency being the amount of flux which can be generated within a core given a specific field strength. While high efficiency is desirable, size should be minimized.
There are two primary core characteristics which affect efficiency: core permeability and core losses. The permeability of a substance is the quotient of a change in magnetic induction divided by the corresponding change in magnetizing force. For example, when a magnetic field having a specific strength induces a relatively large amount of flux within a substance, the substance is said to have a high permeability. When a field induces a relatively small amount of flux within a substance, the substance is said to have a low permeability.
The term core losses is used to refer collectively to two different types of energy loss which are generally referred to as eddy current and hysteresis losses. Hysteresis losses are caused by the reality that it takes some energy to change the magnetic state of a substance. For example, when a magnetic substance is placed within a magnetic field, field energy in the form of flux is guided through the substance. A first portion of field energy is stored and is wholly recoverable from the substance when the substance is removed from the field. A second portion of field energy is converted to heat as a result of work required to magnetize the substance and begin flux flow. Hysteresis losses comprise this second portion of field energy.
Eddy current losses result from an electric field and consequent circulatory currents induced in a core by time varying fluxes. Whenever magnetic flux within a substance changes, an electric field is generated within the substance. When the substance is conductive, the electric field causes currents within the substance which are referred to as eddy currents. Eddy currents cause substance heating and subsequent eddy current losses which are proportional to the square of the eddy current multiplied by the substance resistance.
One common core type is a core formed of powdered iron material. To form this type of core, powdered iron material is fed into a mold, compressed under extremely high pressure and then sintered or resin bonded into a finished "compact". The sintering or resin bonding processes hold the material in the compact form. There are many advantages to powdered iron cores including minimum wasted core material and extremely high magnetic permeability. In addition, resin bonded compacts generate minimal eddy current losses. Moreover, sintering can hold powdered iron in a small core configuration.
Unfortunately, compacting force within the powdered material drops off dramatically with distance from the compacting surface, resulting in lower effective density within the core. This is especially true of resin bonded compacts. Generally, as the effective density of a core decreases the relative size of the core increases. In addition, loosely packed cores have minimal mechanical strength. Moreover, while sintered compacts are relatively small, sintered compacts generate excessive eddy current losses.
Another common core type which overcomes some of the shortcomings of the powdered iron core is a laminated core. A laminated core is formed by stacking a plurality of electrical steel laminations together along the length of the core, each lamination being a flat member having oppositely facing first and second surfaces, each surface having the general shape and area of a transaxial slice of the core. An electrically insulating core plating layer is provided on each of the first and second surfaces of each lamination. When stacked together, core plating provides an electrical barrier between adjacent laminations without impeding magnetic flux within the core. While eddy currents still exist within each lamination, the electrical barriers impede greater eddy currents from flowing throughout the core.
Because different materials have different permeabilities and different hysteresis loss characteristics, the easiest way to increase permeability and decrease hysteresis losses in a laminated core and thereby enhance core efficiency is to choose substances for forming laminations which are known to have a high permeability and low hysteresis losses. Similarly, eddy current losses can also be further minimized by choosing laminate substances having characteristically low eddy current losses. As a starting point, usually core materials are limited to metals which are generally permeable and have relatively low losses. Core materials are also usually doped with a loss inhibitor such as silicon to minimize core losses.
To enhance permeability and reduce losses even further, virtually all substances used to form laminations are subjected to various processes including at least one annealing process which is performed either by the material manufacturer or the core manufacturer. During an annealing process a sheet of metallic substance is held at an elevated temperature for the duration of a specified period in order that metastable high permeability and low loss characteristics go into thermal dynamic equilibrium. After the specified period, the substance is cooled slowly back to room temperature, the metastable conditions becoming permanent characteristics of the substance. The metallic sheet or coil may be coated with core plate material as a coil or sheet or the individual laminations may be coated after the laminations have been stamped from the sheet or coil. Depending upon the properties of the metallic substance and the machine manufacturer's processes the laminations may or may not be given a final anneal before assembly to form the electrical machine cores.
Even where an annealing process is precisely controlled, resulting substance permeability and losses cannot be precisely determined until after annealing. This is because initial substance characteristics, even for a specific grade of substance, are not always the same and disparate initial characteristics are reflected in the annealed substance. In addition, even where two substances have similar initial characteristics, the annealing process can affect the substances differently. For this reason, instead of annealing to generate substances with precise characteristics, it is common practice within the industry to anneal with a target range of substance characteristics as a goal, the target range defined by a minimal acceptable permeability and maximum acceptable core losses. After annealing, the substance characteristics are determined using substance samples which are annealed along with the lamination material. The samples are tested (e.g. according to an Epstein test as well known in the art) after annealing by industry prescribed procedures for permeability and losses to ensure that substance characteristics at least meet the minimum requirements of the target range. Other motor components are then designed in light of the minimum substance requirements to provide the minimal required motor operating characteristics.
Unfortunately, while conventional laminated cores as described above overcome some of the problems associated with powdered iron cores, they too have several shortcomings. For example, because laminations are stamped out of sheet material, some lamination scrap is generated and wasted. In addition, because annealed lamination material has disparate characteristics, designers have to configure cores for worst case conditions including minimum allowable permeability and maximum allowable core loss. This constraint causes inefficiencies. For example, some finished laminations might not meet the minimum core loss and permeability requirements and therefore might have to be scrapped. Other lamination material might far exceed the minimum core loss and permeability requirements and thus provide excessive core capability. Moreover, core plated laminations are relatively expensive and therefore increase motor cost.
Furthermore, in many motors, the primary path for dissipating core heat is thermal conductivity through the back iron of the motor stator. While silicon and similar materials reduce core losses, silicon decreases thermal conductivity thereby "bottling up" heat within the core which reduces rated motor output and can require more expensive insulation or a separate heat dissipating mechanism, both options increasing motor costs considerably.
Moreover, for high speed motors, the rotor often has massive structures (e.g. rotor bars or windings in other types of electromotive machines) which are constrained radially solely by the rotor tooth tips or slot bridges. Unfortunately, annealing and other material processing which results in good electrical performance of steel laminates typically lessen laminate mechanical strength.
In addition, some motors require specially designed and relatively expensive cores to enable the motors to operate in special applications. One special application requires a strong axial force which can move a rotor axially within a stator cavity from a first position into a second position. For example, electric hoist motors often require axial rotor movement. One common electrical hoist motor designed to facilitate axial rotor movement includes a rotor and a stator which together define a conical air gap, the stator cavity narrower at a first end than at a second end, the rotor including first and second ends, the second end wider than the first end. A conical brake drum is provided at the second end of the stator cavity and a similarly shaped brake shoe is affixed to the rotor shaft between the second rotor end and the drum. An axial coil spring at the first rotor end biases the rotor and shaft out of the stator into a first position wherein the shoe is received in the drum thereby impeding rotor rotation. When the motor is energized, a strong axial force is formed in the air gap due to the conical geometry of the gap. This force compresses the axial coil spring thereby releasing the brake, moving the rotor into a second position within the stator and allowing rotor rotation. When power is removed, the spring decompresses and the rotor is forced once again into the first position with the brake impeding rotor rotation. This type of conical core design is expensive. This is because a large number of dies are required to stamp out laminations for forming conical cores which in turn form the conical air gap.
For all of these reasons, it would be advantageous to have versatile core "building blocks" which could be combined to form a core structure having selected core characteristics including permeability, core loss, mechanical strength and heat dissipation and which could be formed inexpensively.